1. Field of the Invention
This invention relates to exhaust gas recirculation (EGR) systems for internal combustion engines and especially to an EGR valve for turbocharged or supercharged diesel engines.
2. Description of Prior Developments
It is well known to recirculate engine exhaust gas for supplemental combustion in order to reduce the level of pollutants exhausted into the atmosphere. In spark-ignition engines, the air intake passage of the engine is typically at a subatmospheric pressure during engine operation. The subatmospheric pressure is often used for actuating an exhaust gas recirculation valve. This practice is well understood and documented in the prior art.
In turbocharged or supercharged engines, the air intake passage is typically above atmospheric pressure due to the air compressing action of the turbocharger or supercharger compressor on the intake air. Although the exhaust gas recirculation valve can be operated with a pressure differential between the air intake passage and the exhaust gas passage, valve operation is difficult or ineffective during times when the intake air passage pressure exceeds the exhaust gas pressure. In particular, the higher pressure of the intake air prevents the lower pressure exhaust gas from entering the intake air passage.
An example of a known EGR system is disclosed in U.S. Pat. No. 4,484,445 to Gillbrand. A super-charged engine includes a driver-operated throttle valve located in the air intake passage to generate air flow in two control lines connected to the passage at closely spaced points upstream from the valve. As the valve opens and closes, the pressures at the points where the control lines connect to the passage vary so that a pressure differential is established between the two control lines.
The respective lines are connected to opposite sides of a diaphragm-type actuator for a gas recirculation valve, such that the valve can be opened or closed by the pressure differential across the two lines. This approach is inappropriate for a compression ignition (diesel) engine since there is no throttle valve in the diesel system.